1. Field of the Invention
The present invention relates to replaceable filter elements. Specifically, the present invention relates to an outside-in flow replaceable filter element with integral annular trap.
2. Background Information
Hydraulic systems are frequently used in heavy machinery, including cranes, backhoes, demolition shears, bulldozers, and the like. In such hydraulic systems, it is important to keep the hydraulic fluid free of debris. Consequently, filter units or filter assemblies have been incorporated in the hydraulic systems to filter debris from the hydraulic fluid. Fuel systems and lubrication systems have also incorporated filter assemblies to clean the working fluid (with the fuel system being the only systems that do not typically re-circulate the working fluid).
It is common to form the filter assembly with a replaceable filter element. One common configuration is a tubular filter media in which the fluid being cleaned flows in a radial direction through the tubular pleated media. The direction of flow in such a structure defines the filter element as an outside-in (flow of fluid being cleaned is inward radial direction) or an inside-out (flow of fluid being cleaned is outward radial direction) filter structure. Examples of the tubular structure type filter elements include the K series element sold by Schroeder Industries, LLC, with the direction of flow for this element being defined by the associated assembly. From an operational standpoint there are certain advantages to outside-in flow and other advantages with inside-out flow. Namely with outside-in flow the pleat structure is more stable at higher pressure when constructed with a traditional fan pleat. This structure also exhibits higher dirt holding capacity as compared to similar inside out designs, since the media pleats are uniformly exposed to the system flow which results in a more uniform distributed deposition pattern. Alternatively with inside-out flow there is an advantage in that larger particles on the up-stream side of the filter remain in the center and are easily removed with the filter element, whereas in the outside in configurations such particles can, in theory, contaminate the system by falling or migrating past the filter location during filter element replacement. This contamination may occur after system flow stops which may allow particles on the outer most media layer to become dislodged and further contaminate the upstream fluid or settle within the filter housing. If this condition occurs and the element is removed from the filter housing (i.e. replacement of the filter element); the dislodged particles in the upstream fluid may contaminate the fluid downstream of the filter assembly.
U.S. Pat. No. 6,322,697 (the '697 patent) explains that contamination “is an important consideration relating to filter assemblies having replaceable filter elements. For example, prior art systems having replaceable filter elements can experience the accumulation of contaminants within the reusable housings that cover the replaceable filter elements. If this occurs, the “clean” side of new filter elements inserted within the contaminated housings can immediately become contaminated. There is a need in the art to prevent this type of contamination from occurring.” The '697 patent discloses six separate embodiments of the invention to address this problem using a cap surrounding an annular “trap” area. However, in the cap designs in the '697 patent the openings in the top cap are aligned with the annular area and necessarily will have a reduced area than that of the trap forming area (due to the outwardly extending portions of the cap), and this can form a restriction for flow. There is a need in the industry for a simple, efficient, tubular filter element that combines the advantages of inside out-flow and outside-in flow, and which does not introduce flow limiting characteristics.